Smoke detectors of the photoelectric type are often used in fire alarm system installations, where a plurality of remote detectors are connected to a central control panel. It is often desirable in such systems to provide means for providing a standby voltage on the signal lead, which is appreciably lower than the alarm voltage, with means at the control panel for indicating the absence of the standby voltage. It has also been desirable to provide means for indicating the condition of the smoke detector light source.
Smoke detectors of the photo-electric type are often provided with a compensating cell connected in series with the smoke detector cell. The compensating cell is constantly exposed to light from the detector light source, whereas the detector cell is exposed to light from the light source only when smoke appears in the light beam and reflects smoke onto the detector cell. The output voltage for actuating an alarm is taken from the junction between the cells.